Posterior Fossa Level: Brainstem and Cranial Nerve Nuclei

The posterior fossa level contains all the structures located within the skull below the tentorium cerebelli and above the foramen magnum. These structures are derivatives of the embryonic mesencephalon, metencephalon, and myelencephalon and include portions of all the systems discussed in other chapters. The major structures of this level are the brainstem, cerebellum, and segments of cranial nerves III-XII before their emergence from the skull. The brainstem, the core of the posterior fossa level, is a specialized rostral extension of the embryonic neural tube that preserves, even in the mature state, many of the longitudinal features of the spinal cord and provides for segmental functions of the head. This chapter describes the general anatomy of the posterior fossa, the anatomy and functions of cranial nerves III-XII, and the internal anatomy of the medulla, pons, and midbrain.